Most of the current electric bicycles are driven by a wheel hub motor, and in the wheel hub driving manner, an electrical motor is disposed on a front wheel or a rear wheel of the bicycle, which causes the center of gravity of the whole bicycle to deflect forward or rearward, thus does not conform with the common riding manner, and the speed of the bicycle is completely controlled by a rotation handle, therefore large traffic hidden dangers exist. At the same time, such wheel hub type mechanical structure increases the difficulties in assembling, and examining and repairing, which virtually increases the using cost and failure rate of the electric bicycle. A center-mounted control system refers to releasing the electrical motor from the front wheel or the rear wheel of the bicycle, the center-mounted control system employs a high-speed brushless direct-current motor instead of the wheel hub motor, and the high-speed brushless direct-current motor is mounted on a crank shaft of the bicycle. The center-mounted control system integrates a speed sensing (or a torque sensing) with the electrical motor control to control the normal driving of the whole bicycle, thus is a core component for the power assist bicycle. Using of the center-mounted control system may overcome the disadvantages of the conventional electric bicycle, has an excellent riding effect, and will undoubtedly become the developing direction of the electric bicycles.
In addition, most of the center-mounted control systems for the power assist bicycle available in the market use a speed transducer for feeding back the speed (that is the speed of a foot pedal being stepped by an rider) to control a driving force of the electrical motor. The driving force outputted by the electrical motor is directly proportional to the speed, thereby realizing power assistance. A center-mounted system based on speed detection has a certain power assistance effect under a normal road condition, however in the case that the rider rides against the wind, or rides on a road condition with a high resistance and a heavy load such as an upslope, the driving force outputted by the electrical motor is small because of the low riding speed, thus the power assistance effect cannot be realized, and the riding effect is bad, therefore the center-mounted system based on speed detection cannot be accepted by the mainstream market. Therefore, the best solution is to employ a torque transducer to feed back a force of the rider stepping on the foot pedal, so as to control the driving force of the electrical motor, and the larger the torque is, the larger the driving force is.
However in the conventional torque detection, a human power is converted into an angular displacement, two magnetic rings are staggered, and an angle corresponding to the relative motion is detected via a Hall sensor, thus the torque of the rider stepping on the foot pedal can be detected. However for the torque detection based on the magnetic rings and the Hall sensor, the mechanical structure is complicated, and the requirement for the installation accuracy is very high, thus disadvantages are apt to occur in a later period, for example a measuring error may be caused by falling of the bicycle. Besides, the conventional center-mounted system based on the torque detection also has the following problems. 1. The torque transducer is directly arranged on the crank shaft, for example, a torque sensing system arranged on a crank shaft of a bicycle is described in the Patent Application No. 201010548510.4. In this system, the transmission process of the torque when a left-side foot pedal is hardly stepped by the rider is from the left-side foot pedal to the crank shaft, to the torque transducer, and then to a chain ring, that is, the torque transducer can detect the torque before the chain ring rotates, thus may timely drive the electrical motor to assist the human-powered riding. However in the case that the force is applied on a right-side foot pedal, the transmission process of the torque is from the right-side foot pedal to the crank shaft and then to the chain ring, thus when the chain ring already rotates, the torque transducer cannot detect the torque, thus cannot drive the electrical motor. An existing solution is to simulate a left torque and a right torque via a software after the bicycle being started, therefore the existing torque transmission method cannot realize a zero-speed start, which has a had riding effect, moreover, most people are accustomed to starting the bicycle with the right foot, thus according to the above analysis, such solution has no advantage at all. 2. The electrical motor is directly educed through a reduction gear mechanism to be connected to the chain ring, thus when the speed of the rider stepping on the foot pedal is greater than the speed of the electrical motor, the human force may drive the electrical motor to rotate, which increases the load of the human body. Particularly, when a power source of the center-mounted system is closed, the rider may bear a rotating load of the electrical motor during the riding process, thus riding such bicycle is more strenuous than riding an ordinary bicycle. 3. For the signal processing mode of the existing center-mounted system, detected torque signals are generally performed with only simple signal processing and then are all sent to a controller, therefore the dependency of the center-mounted system on the controller is increased to a certain extent, and meanwhile a certain demand for function of the controller is put forward, which reduces a selection model range of the controller, and weakens the versatility of the center-mounted system. 4. The existing center-mounted system employs the Hall sensor to detect only a rotation speed signal of the chain ring, and cannot realize a double control of the rotation speed and a rotation direction of the chain ring, thus cannot assist the electrical motor well in controlling, and misoperation, or bicycle flying-off and other dangerous phenomena may even occur.